1. Field of the Invention
The present invention relates to an electric system for an electric vehicle comprising a main battery for driving the vehicle and an auxiliary battery for accessories in which drive power is supplied via an inverter to an AC motor for driving wheels.
2. Description of the Related Art
FIG. 8 shows a conventional electric system for an electric vehicle having a main battery as its power supply. In this figure, reference numeral 1 power supply. In this figure, reference numeral 1 designates a main battery consisting of a series of a number of unit batteries 100; 2, a main switch; 3, fuse; 4, an inverter for driving a motor which is PWM (pulse width modulation) controlled; 5, an AC motor. In this electric vehicle, the AC motor 5 is connected to wheels of the vehicle through reduction gears and so on (not shown). Further, reference numeral 6 designates an auxiliary battery for accessories; 7, a charger. The charger 7 comprises a charging circuit for charging the auxiliary battery 6 and an initial charging circuit for charging an input capacitor in the inverter 4 which is provided to absorb higher harmonic current when the inverter 4 operates. Reference numeral 8 also denotes a switch for charging.
Reference numeral 200 denotes accessories such as lamps and a car radio and the like which use the auxiliary battery 6 as their power supply. In the case of charging the auxiliary battery 6, the charging switch 8 is closed and then the auxiliary battery charging circuit in the charger 7 operates.
In the system described above, a three-phase inverter employing transistors is used as the inverter 4 and an induction motor is widely used as the motor 5.
FIG. 9 shows an example of a circuit when a three-phase inverter is used as the inverter 4 in which reference numeral 41 denotes transistors, 42 denotes diodes, each of which is connected in antiparallel with each one of the transistors 41, and 43 denotes the input capacitor. Reference numeral 44 denotes AC output terminals connected to the motor 5, and 45 denotes DC input terminals connected to the main battery 1 through the fuse 3 and the main switch 2.
FIG. 10 shows a constitution of the charger 7 in FIG. 8. In this example, the charger 7 is constituted by a DC-DC converter. In FIG. 10, reference numeral 70 denotes the auxiliary battery charging circuit and 700, the initial charging circuit for the input capacitor 43 in the inverter 4. In this auxiliary battery charging circuit 70, reference numeral 71 denotes an input capacitor; 72, an inverter; 73, an insulating transformer; 74, a diode rectifier; 75, a smoothing circuit; 76, output terminals connected to the auxiliary battery 6; 77, input terminals connected to the main battery 1 through the switch 8 for charging. Here, the inverter 72 is constituted by a single-phase transistor inverter which is similar to the three-phase transistor inverter 4 in a main circuit.
On the other hand, in the initial charging circuit 700, reference numeral 701 denotes an input capacitor; 702, an inverter having the same constitution as that of the inverter 72; 703, an insulating transformer having a turns ratio of the primary windings to the secondary windings equal to 1; 704, a diode rectifier; and 706, output terminals connected to the input capacitor 43 in the inverter 4. The inverter 702 in the initial charging circuit 700 is operated only when the inverter 4 is started and initially charges the input capacitor 43. Further a protective fuse may be provided on the input side or on the output side of the charger 7, if necessary.
In the electric system for an electric vehicle set forth above, the voltage of the auxiliary battery 6 is normally 12 V similarly to automobiles having an internal combustion engine, while the voltage of the main battery 1 is not less than 200 V and might be over 20 times the voltage of the auxiliary battery 6. In such systems, as the input voltage (main battery voltage) applied to the charger 7 for the auxiliary battery 6 is over 20 times the output voltage (auxiliary battery voltage), electrical insulation between the input and output terminals is required for a safety purpose. Accordingly, the conventional charger 7 was forced to use the DC-DC conversion system as mentioned above which included the inverter for inverting DC input power to AC power, the rectifier for converting the AC power to DC power, and the smoothing circuit. This results in that the conventional charging system becomes not only complicated and bulky in structure, but also expensive.
Since a charger having an auxiliary battery charging circuit, an initial charging circuit for an input capacitor of an inverter and so on is mounted on a vehicle, it is required to be small in size and light in weight like a main circuit for controlling a motor. At the same time, a low cost and highly efficient charging system is desired.